Search Results - (Author, Cooperation:G. W. Crabtree)

2016-05-21

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

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Natural Sciences in General

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1089-7550

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Physics

Low-field dc magnetization measurements in fields between 0.5 and 40 G are reported on a single crystal of nominal composition Tl2Ba2Ca2Cu3O10. It is observed that Hc2, determined from the diamagnetic onset temperature, is quite anisotropic. Extrapolating Hc2 to zero field yields zero field superconductive transition temperatures of Tc=118 K for H parallel to the c axis and Tc=111 K for H perpendicular to the c axis. This apparent inconsistency is thought to be due to the variation in stacking sequence of the Cu-O sheets.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

In this article we describe a simple SQUID magnetometer system useful for the magnetic characterization of high temperature superconductors. The system employs a commercial rf SQUID probe with associated electronics located in a 4He gas exchange cryostat. The system is equipped with a copper coil dc magnet for fields up to 50 Gauss. The cryostat is surrounded by a μ-metal shield that keeps the remanent field to less than 20 mGauss. The apparatus has been used extensively to measure both the superconducting transition temperature and width as well as the relative quantities of magnetic flux expulsion and exclusion. It has therefore served as an important first step in the evaluation of sample quality.

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1089-7550

AIP Digital Archive

Physics

A high-resolution magneto-optical imaging technique is used to reveal the formation of twins occurring during a martensitic phase transition at ∼105 K in La2/3Ca1/3MnO3 (LCMO) films grown on SrTiO3 (STO) substrates. The magnetic contrast arises due to the magneto-elastic tilts of the Mn- magnetic moments in the twins. Different magnetic structures are found in LCMO films grown on MgO, NGO, and LAO substrates showing the importance of the substrate material for the manganite film properties. © 2000 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We report further measurements of the temperature dependence of the piezotube coefficient for material PZT-5A. These new results support our earlier findings that the temperature dependence of the piezoresponse is linear to within 10% and is reduced by a factor of 5.5 on decreasing temperature from 300 to 4.2 K. Our new data imply that the piezocoefficient temperature dependence is reproducible for tubes of various sizes, even when the room temperature coefficients differ by more than 20% from their published values. © 1994 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We have calibrated the displacement/voltage (A(ring)/V) response of our piezoelectric scanning tube (PZT-5A) by imaging graphite at over 40 temperatures between 4 and 300 K. We have also calibrated the (A(ring)/V) response as a function of voltage up to 220 V at room temperature, imaging a gold-plated diffraction grating. We find that the temperature dependence of the (A(ring)/V) response is linear to within 10% and is reduced by a factor of 5.5 on decreasing temperature from 300 to 4.2 K. The near linearity with temperature of the (A(ring)/V) response makes the PZT-5A lead zirconate titanate composition a convenient choice for low temperature scanning tunneling microscope piezo tube elements.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The temperature dependence of the energy gap of bulk polycrystalline Ba1−xKxBiO3 (BKBO) was measured with a low-temperature scanning tunneling microscope in the point contact mode. At 4.2 K, the tunneling conductance shows nearly ideal BCS behavior, however at higher temperatures, the lifetime broadening term Γ was required to fit the data. Γ(T) displays a surprising T3 temperature dependence and the magnitude is significantly larger than the calculated theoretical recombination times. Comparisons with other conductance data obtained from grain boundary and sandwich-type BKBO junctions show similar results. The large scattering rates for BKBO may be important for the utility of this material in SIS mixers.

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1089-7550

AIP Digital Archive

Physics

Proximity effect gaps were observed consistently in Nb/Au, Bi2Sr2CaCu2O8+y/Au, and HgBa2CuO4/Ag bilayers using a low temperature scanning tunneling microscope. The Nb/Au conductance curves [dI(V)/dV vs V] were fit by the Arnold model, using the accepted energy gap value of Nb, 1.55 meV. The application of gold and silver overlayers allowed vacuum tunneling and topographic images of BSCCO single crystals and the HgBa2CuO4 polycrystalline sample. Cleavage steps of varying sizes (up to 1500 A(ring)) in the BSCCO/Au sample were observed, and gaplike features were found on such steps. Consistent, reproducible energy gaps were also found in the Hg-compound sample. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have measured ac magnetoresistance in fields up to 8 T in a variety of YBa2Cu3O7−δ single crystals containing (a) twin boundaries only in one direction, (b) multi-twin domain structure, and (c) growth steps. We find two components of the resistance: an angular dependent Lorentz force induced "excess resistivity'' superimposed on an angular independent resistivity. The temperature and magnetic field dependence of the angular-dependent resistivity exhibit a strong correlation with crystal defects, whereas the angular-independent resistivity remains virtually unchanged. In addition, we find crystals exhibiting both ohmic and non-ohmic behavior in their I-V characteristics depending on crystal imperfections.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0〈x〈2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Measurements of the magnetization hysteresis of the same crystal in the twinned and untwinned state reveal that pinning due to twinning planes may contribute a substantial, anisotropic fraction of the observed critical currents. A linear scaling between magnetization hysteresis and sample dimension shows that the twinned and untwinned crystal does not exhibit any subgranular effects.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Heat capacity, resistivity and magnetic susceptibility data have been used to study electronic and magnetic properties of REBa2Cu3O7−x compounds. Neutron diffraction on the Ho compound shows identical crystal structure to YBa2Cu3O6.8. All the compounds are superconducting at Tc=(93±2) K. Magnetic transitions are observed in the heat capacity data at Tm=2.2, 0.95, 0.17, and 0.59 K for RE=Gd, Dy, Ho, and Er, respectively, and the magnetically ordered state is found to coexist with superconductivity. Estimates have been obtained for the magnetic moment in the crystal field ground state and the energy splitting of the first excited state crystal field level.

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1089-7550

AIP Digital Archive

Physics

Low-field magnetization measurements in flelds from 0 to 100 G at temperatures from 4.2 to 100 K on single crystals of YBa2 Cu3 O7−δ are presented. The crystals have sharp inductive transitions at 91.5 K, occasionally with additional structure suggesting a second phase with a lower Tc . For all crystals in all directions the Meissner effect is a small fraction of the shielding effect. Low-field measurements of Hc2 within 1 K of Tc show a linear temperature dependence with a slope of 80 G/K, much smaller than observed at higher field.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We report the first observation of a field-dependent mass in a hybridizing f-electron system. CeB6 is an ordered moment heavy fermion system with an electronic specific heat coefficient γ of order 225–300 mJ/mole K2. Using the de Haas–van Alphen effect at temperatures as low as 60 mK in steady magnetic fields as large as 22 T, we observe a cyclotron orbit of frequency 8680 T for fields along the [100] direction. The mass of this orbit was measured at eight fixed fields and found to decrease from 18me to 8me as the field increases from 12 to 22 T. The observed Fermi surface is very similar to that of LaB6, indicating that the f-electrons are largely local rather than itinerant in CeB6, a picture confirmed by bandstructure calculations. The observed field dependence of the cyclotron mass is consistent with the low-energy scale of the system as measured, for example, by the Kondo temperature. Our results are compared with Fermi surface observations in other heavy fermion systems.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using magnetization hysteresis loops we have determined the irreversibility line of polycrystalline samples of HgBa2CuO4+δ and Hg0.8Pb0.2Ba2CuO4+δ. The irreversibility line is found at rather high temperatures. It does not show the drastic depression with increasing temperature as is observed in highly anisotropic Bi/Tl-based CuO superconductors. At 77 K the irreversibility field is around 0.3 T, which is substantially higher than in Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10. The advantages of this material for large-scale applications are outlined.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We present a study of the superconducting morphology of the transport current carrying cross section of a 19-filament (BiPb)2Ca2Cu3Ox (Bi-2223) composite conductor using magneto-optical imaging of magnetic flux patterns. In conjunction with electron microscopy on the same sample this technique allows a unique correlation of superconducting and microstructural properties. Direct evidence for enhanced superconducting properties in platelike regions along the silver/Bi-2223 interface and for weak properties near the core of the filaments is obtained. Misaligned grain colonies are found to cause an interruption of the superconducting continuity in the filaments. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using a high-resolution magneto-optical imaging technique, we study the flow of transport currents around cracks in monofilament (BiPb)2Sr2Ca2Cu2Ox composite conductors. Shunting through the Ag clad is directly observed. The current in the silver decays exponentially with distance from the crack as expected in a one dimensional distributed resistance model. At 77 K, the transfer length, λ, is approximately 0.56 mm, implying an interface resistivity of 7.5×10−8 Ω cm2. This transfer length is found to increase slightly with increasing temperature. © 1997 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The magnetization behavior of permalloy films containing a square array of holes has been studied using magnetometry, magneto-optical imaging, and magnetic force microscopy. Two types of domain boundaries are observed. The hole array induces a strongly pinned, complex domain structure on the microscopic level (i.e., within one unit cell of the hole array). We show that magnetization reversal occurs through the nucleation and motion of a domain wall that separates areas of oppositely polarized microscopic domain patterns. These walls display highly mobile kinks that induce the reversal of the microscopic patterns. The interplay between intrinsic anisotropy and pinning by the hole array induces a pronounced anisotropy in the domain patterns and switching mechanism. © 2001 American Institute of Physics.

Electronic Resource

1749-6632

Blackwell Publishing Journal Backfiles 1879-2005

Natural Sciences in General

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Early experimental indications4"6 of the occurrence of a first-order transition in the vortex state of untwinned YBa2Cu3O7_(5 came from a sharply vanishing in-plane resistivity which showed hysteresis along a phase boundary Hm(T) (with H parallel to the c axis of the crystal). However, the ...

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